1. Field of the Invention
The present invention relates to an over-current protection device.
2. Description of the Prior Art
The resistance of PTC conductive material is sensitive to temperature change. Due to such property, the PTC conductive material can be used as current-sensing material and has been widely used in over-current protection devices and circuits. The resistance of the PTC conductive material remains low at room temperature so that the over-current protection device or circuit can operate normally. However, if an over-current or an over-temperature event occurs, the resistance of the PTC conductive material immediately increases to a high-resistance state (over 102 ohm). Therefore, the excessive current is blocked and the objective of protecting the circuit elements or batteries is achieved.
In general, the PTC conductive material contains one or more crystalline polymers and a conductive filler. The conductive filler is dispersed uniformly in the crystalline polymer. The crystalline polymer is mainly a polyolefin polymer such as polyethylene. The conductive filler is mainly carbon black, metal particles such as nickel, gold or silver, and/or ceramic powder such as titanium carbide or tungsten carbide.
The conductivity of the PTC conductive material depends on the content and type of the conductive filler. Generally, carbon black having a rough surface provides better adhesion with the polyolefin polymer, and accordingly, a better resistance repeatability is achieved. However, the conductivity of the carbon black is lower than that of the metal particles, so that there is a trend to replace the carbon black with metal filler. If the metal particles are used as the conductive fillers, their larger specific weight results in a less-uniform dispersion. For example, nickel fillers exhibit weak magnetism, so the filler particles accumulate easily and are not easily dispersed.
To effectively reduce the resistance of the over-current protection device and prevent uneven dispersion of the fillers, the non-conductive ceramic powder or filler is often added to metal filler material. The dispersion can be significantly improved by friction and filling behavior when blending the ceramic powder, polymer and metal filler. That is, the ceramic filler serves as a solid dispersing agent for conductive material. Moreover, since the ceramic powder lacks a rough surface like carbon black and has no obvious chemical function groups, the ceramic powder exhibits poor adhesion with the polyolefin polymer, compared to the adhesion of the carbon black to the polyolefin polymer, and consequently, the resistance repeatability of the PTC conductive material is not well controlled. A coupling agent may be added to the PTC conductive material with the metal filler, so as to improve the adhesion and reacting force between the metal filler and the polyolefin polymer, significantly reduce voids in the conductive material, and increase the resistance repeatability.